1. Field of the Invention
The present invention relates to methods for fabricating superconducting coatings and, more particularly, to a method for fabricating oxide superconducting coatings having a characteristic of weak bond in grain boundaries with high precision and high density by electrophoresis.
2. Description of the Prior Art
Recently, an oxide superconducting material that has a critical temperature (Tc) of liquid-nitrogen temperature 77 K or more has been discovered. Oxide superconducting materials are short in coherence length of carriers and low in carrier concentration, and therefore can be easily put into weak bond in grain boundaries of crystal grains. Taking advantage of the Josephson effect of this weak bond, they are applied to magnetic sensors and optical sensors, which can sense weak energy, or logic elements and transistors, which can be controlled by small input. To progress application and development of such oxide superconducting materials, it is essential to establish techniques by which superconducting coatings can be formed or processed into desired shapes without impairing the superconducting characteristic.
The formation itself of oxide superconducting coatings can be carried out relatively easily by using a thin-film fabrication method such as sputtering, vacuum deposition (reactive deposition, molecular beam epitaxy (MBE), ion cluster beam (ICB), laser deposition), and chemical vapor-phase growth method (RF-plasma, optical CVD, organometallic chemical vapor-phase growth (MO-CVD) or by using a thick-film fabrication technique such as spray pyrolysis, screen printing, and sol gel. However, the sputtering, vacuum deposition, and MO-CVD methods, capable of fabricating high-quality thin films as they are, would result in small areas of fabricated films and moreover necessitate vacuum equipment for the fabrication, thus involving large-scale facilities. On the other hand, the spray pyrolysis, screen printing, sol gel and like methods are, although not suitable for fabricating single-crystal thin films, yet widely used for applications in which polycrystalline materials are utilized, for example, magnetic shields, wire materials, and sensors. In particular, high-temperature superconducting materials are short in coherence length and low in carrier concentration, and therefore have a feature that they may be easily made in weak bond similar to the Josephson junction when a polycrystalline film is fabricated. Taking advantage of this effect, they contribute to the realization of, for example, magnetic sensors, optical sensors, and logic elements to be realized, thus being expected for a wide variety of applications. Despite the above fact, these conventional methods such as the spray pyrolysis, screen printing, and sol gel involve complex chemical processes in film formation, having encountered difficulties in repeatability and reliability of film characteristics. Also, it has been difficult to form films on large-scale, complex-shaped substrates by these methods.
Recently, in view of these problems, a fundamental technique for forming superconducting coatings by electrophoresis has been reported (for example, Appl. Phys. Lett. 55 (1989) 492). This method is, in brief, one in which a superconducting powder dispersed in an organic solvent is deposited on a substrate biased to a negative or positive potential, and thereafter the substrate is sintered by heat treatment to form a superconducting film. This method is expected for a wide variety of applications by virtue of its capability of forming superconducting films on large-scale, complex-shaped substrates.
However, substrates for this method have been provided heretofore by using noble metals such as gold or silver that involve less reactions with high-temperature oxide superconducting materials, while other substrate materials could not afford the superconductivity. Such a limitation on the material of substrates has caused this method to be costly, making it difficult to put the method into practical applications. Moreover, the method has suffered not only from obscurities in the mechanism of fabrication but also from insufficient criteria for controlling the characteristic.
The inventors of the present application of patent have previously proposed a method that allows superconducting coatings to be formed on a copper substrate with its resistance being zero by controlling the conditions except the atmosphere for sintering, using copper as a substrate, which had previously been taken impossible (Japanese Patent Application No. 3191/1990). Disadvantageously, although a film fabricated by the method indeed becomes superconductive, its zero-resistance temperature is in the range of approximately 60 to 80 K in the case of Y-Ba-Cu-O superconducting coatings, thus the resulting film characteristic being not satisfactory.